1. Field of the Invention
The present invention relates to a power supply for a cold-cathode fluorescent lamp (CCFL) system. More particularly, the present invention relates to a power supply topology that delivers evenly-distributed current to each CCFL in a multiple CCFL system. The present invention has particular utility in applications which utilize CCFL technology, for example, display systems found in portable computers, instrumentation, etc.; although other utilities are contemplated herein.
2. Description of Related Art
FIG. 1 depicts a conventional CCFL power supply system 10. The system broadly includes a power supply 12, a CCFL driving circuit 16, a controller 14, a feedback loop 18, and one or more lamps CCFL1, CCFL2. Power supply 12 supplies a DC voltage to circuit 16, and is controlled by controller 14, through transistor Q1. Circuit 16 is a self-resonating circuit, known as a Royer circuit. Essentially, circuit 16 is a self-oscillating dc to ac converter, whose resonant frequency is set by L1 and C1, and N1-N4 designate transformer windings and number of turns of the windings. In operation, transistors Q2 and Q3 alternately conduct and switch the input voltage across windings N1 and N2, respectively. If Q2 is conducting, the input voltage is placed across winding N1. Voltages with corresponding polarity will be placed across the other windings. The induced voltage in N4 makes the base of Q2 positive, and Q2 conducts with very little voltage drop between the collector and emitter. The induced voltage at N4 also holds Q3 at cutoff. Q2 conducts until the flux in the core of T1 reaches saturation.
Upon saturation, the collector of Q2 rises rapidly (to a value determined by the base circuit), and the induced voltages in the transformer decrease rapidly. Q2 is pulled further out of saturation, and V.sub.CE rises, causing the voltage across N1 to further decrease. The loss in base drive causes Q2 to turn off, which in turn causes the flux in the core to fall back slightly and induces a current in N4 to turn on Q3. The induced voltage in N4 keeps Q3 conducting in saturation until the core saturates in the opposite direction, and a similar reversed operation takes place to complete the switching cycle.
Although the inverter circuit 16 is composed of relatively few components, its proper operation depends on complex interactions of nonlinearities of the transistors and the transformer. In addition, due to variations in C1, Q2 and Q3 (typically, a 35% tolerance between components of this type) circuit 16 is not easily adapted for parallel transformer arrangements, since any duplication of the circuit 16 will produce additional, undesirable operating frequencies which may resonate at certain harmonics. This effect produces a "beat" in the CCFLs, which is a noticeable, and undesirable effect. Even if the tolerances are closely matched, because circuit 16 operates in self-resonant mode, the undesirable beat effects cannot be removed as any duplication of the circuit will have its own unique operating frequency.
In the arrangement shown in FIG. 1, power is supplied to the lamps, CCFL1 and CCFL2, by transformer T1. Each lamp in the system is connected in parallel, and driven by the impedance of Co1 and Co2, respectively. Ideally, Co1 and Co2 are identical so that current is evenly divided between CCFL1 and CCFL2, although, as will be described more fully below, variations within each CCFL can greatly influence the current drawn along each loop. Feedback circuit 18 includes sense resistor R.sub.S, which provides feed back to the controller 14, which in turn, regulates the power input into circuit 16 (via Q1). Importantly, in the topology shown in FIG. 1, only the output current I.sub.OUT is sensed by R.sub.S. Also, as noted above, circuit 16 is not very well adapted to a multiple configuration, thus only one power supply transformer (T1) exists for both lamps, or for that matter, any number of lamps. Thus, the system 10 of FIG. 1 is incapable of determining if any unbalanced condition exists in any individual CCFL loop, since only the output current is sensed. Moreover, any unbalanced impedance along the two loops (Co1, CCFL1 and Co2, CCFL2, respectively) creates unbalanced current through each CCFL, which significantly degrades the expected life of the lamps and the system overall.
Similar CCFL driving systems can be found in U.S. Pat. No. 5,615,093 issued to Nablant; U.S. Pat. No. 5,430,641 issued to Kates; U.S. Pat. No. 5,619,402 issued to Liu; U.S. Pat. No. 5,818,172 issued to Lee; and U.S. Pat. No. 5,420,779 issued to Payne. Each are incorporated herein by reference as disclosing circuit topologies of a type similar to that shown in FIG. 1. Each of these references suffer from similar and/or additional drawbacks as discussed above in reference to the system 10 shown in FIG. 1.
Thus, there exists a need to overcome the aforementioned drawbacks of conventional driving circuits and provide a driving circuit for multiple CCFL systems that overcomes the provides feedback control of each individual lamp in the system, thereby permitting a balanced current state to exist in all lamps in the system, which significantly increases the overall life expectancy of each lamp and the system overall. Moreover, there exists a need to provide a CCFL driving circuit that is relatively simple to implement, and does not suffer from the aforementioned drawbacks when applied to a multiple CCFL system.